Recently, electric vehicles, hybrid vehicles and fuel-cell vehicles have attracting attention as environmentally friendly vehicles. Some hybrid vehicles have already been commercialized.
A hybrid vehicle has, in addition to a conventional engine, a dc power source, an inverter and a motor driven by the inverter as power sources. Specifically, power is obtained by driving the engine and, in addition, dc voltage from the dc power source is converted to ac voltage by the inverter, and the motor is rotated by the converted ac voltage, whereby power is obtained. An electric vehicle has a dc power source, an inverter and a motor driven by the inverter as power sources.
Japanese Patent Laying-Open No. 2001-295676 discloses torque limitation by detecting a state of slip using angular acceleration of a driving shaft in a hybrid vehicle.
FIG. 20 is an illustration showing a slipping state and a gripped state of a vehicle.
Referring to FIG. 20, when a vehicle travels and runs upon a projection on the road surface, for example, upon a road shoulder or an object fallen on the road, the tire is brought to a slipping state, in which the tire rotates idle, immediately thereafter at time point t1. In the slipping state, there is no surface resistance, and therefore, when the wheels are rotated continuously with the same torque, the rotation speed of the wheels increases.
The output (power) is in proportion to torque×rotation speed, and therefore, when a slip occurs, much electrical power (power) is consumed in a motor that drives wheels. Consequently, control is imposed such that more power is supplied to the motor.
On the contrary, at time point t2 after the slipping state, the tire again comes into contact with the road surface, and the rotation speed of the tire decreases abruptly, because of friction from the road surface. As the rotation speed of the tire decreases, the rotation speed of the motor also decreases abruptly.
It is also proposed, in a hybrid vehicle or the like, to boost the dc voltage from the power source by a boosting converter, to convert the boosted dc voltage by the inverter to an ac voltage and to drive the motor thereby.
In such a configuration including a boosting converter, when the rotation speed of the motor decreases abruptly, excessive electrical power would be supplied from the boosting converter to the inverter. Therefore, it becomes necessary to lower the target voltage of the boosting converter.
Japanese Patent Laying-Open No. 2001-295676 does not disclose any control of the boosting converter at the transition from the slipping state to the gripped state.